Editing IPCC:AR6/SRCCL/Chapter-4 (section)

=== 4.8.4 Sustainable forest management (SFM) and CO2 removal (CDR) technologies ===

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While reducing deforestation and forest degradation may directly help to meet mitigation goals, SFM aimed at providing timber, fibre, biomass and non-timber resources can provide long-term livelihood for communities, reduce the risk of forest conversion to non-forest uses (settlement, crops, etc.), and maintain land productivity, thus reducing the risks of land degradation (Putz et al. 2012 <sup>[[#fn:r1071|1071]]</sup> ; Gideon Neba et al. 2014 <sup>[[#fn:r1072|1072]]</sup> ; Sufo Kankeu et al. 2016 <sup>[[#fn:r1073|1073]]</sup> ; Nitcheu Tchiadje et al. 2016 <sup>[[#fn:r1074|1074]]</sup> ; Rossi et al. 2017 <sup>[[#fn:r1075|1075]]</sup> ).

Developing SFM strategies aimed at contributing towards negative emissions throughout this century requires an understanding of forest management impacts on ecosystem carbon stocks (including soils), carbon sinks, carbon fluxes in harvested wood, carbon storage in harvested wood products, including landfills and the emission reductions achieved through the use of wood products and bioenergy (Nabuurs et al. 2007 <sup>[[#fn:r1076|1076]]</sup> ; Lemprière et al. 2013 <sup>[[#fn:r1077|1077]]</sup> ; Kurz et al. 2016 <sup>[[#fn:r1078|1078]]</sup> ; Law et al. 2018 <sup>[[#fn:r1079|1079]]</sup> ; Nabuurs et al. 2017 <sup>[[#fn:r1080|1080]]</sup> ). Transitions from natural to managed forest landscapes can involve a reduction in forest carbon stocks, the magnitude of which depends on the initial landscape conditions, the harvest rotation length relative to the frequency and intensity of natural disturbances, and on the age-dependence of managed and natural disturbances (Harmon et al. 1990 <sup>[[#fn:r1081|1081]]</sup> ; Kurz et al. 1998 <sup>[[#fn:r1082|1082]]</sup> ). Initial landscape conditions, in particular the age-class distribution and therefore carbon stocks of the landscape, strongly affect the mitigation potential of forest management options (Ter-Mikaelian et al. 2013 <sup>[[#fn:r1083|1083]]</sup> ; Kilpeläinen et al. 2017 <sup>[[#fn:r1084|1084]]</sup> ). Landscapes with predominantly mature forests may experience larger reductions in carbon stocks during the transition to managed landscapes (Harmon et al. 1990 <sup>[[#fn:r1085|1085]]</sup> ; Kurz et al. 1998 <sup>[[#fn:r1086|1086]]</sup> ; Lewis et al. 2019 <sup>[[#fn:r1087|1087]]</sup> ). In landscapes with predominantly young or recently disturbed forests, SFM can enhance carbon stocks (Henttonen et al. 2017 <sup>[[#fn:r1088|1088]]</sup> ).

Forest growth rates, net primary productivity, and net ecosystem productivity are age-dependent, with maximum rates of CO <sub>2</sub> removal (CDR) from the atmosphere occurring in young to medium-aged forests and declining thereafter (Tang et al. 2014 <sup>[[#fn:r1089|1089]]</sup> ). In boreal forest ecosystem, estimation of carbon stocks and carbon fluxes indicate that old growth stands are typically small carbon sinks or carbon sources (Gao et al. 2018 <sup>[[#fn:r1090|1090]]</sup> ; Taylor et al. 2014 <sup>[[#fn:r1091|1091]]</sup> ; Hadden and Grelle 2016 <sup>[[#fn:r1092|1092]]</sup> ). In tropical forests, carbon uptake rates in the first 20 years of forest recovery were 11 times higher than uptake rates in old-growth forests (Poorter et al. 2016 <sup>[[#fn:r1093|1093]]</sup> ). Age-dependent increases in forest carbon stocks and declines in forest carbon sinks mean that landscapes with older forests have accumulated more carbon but their sink strength is diminishing, while landscapes with younger forests contain less carbon but they are removing CO <sub>2</sub> from the atmosphere at a much higher rate (Volkova et al. 2017 <sup>[[#fn:r1094|1094]]</sup> ; Poorter et al. 2016 <sup>[[#fn:r1095|1095]]</sup> ). The rates of CDR are not just age-related but also controlled by many biophysical factors and human activities (Bernal et al. 2018 <sup>[[#fn:r1096|1096]]</sup> ). In ecosystems with uneven-aged, multispecies forests, the relationships between carbon stocks and sinks are more difficult and expensive to quantify.

Whether or not forest harvest and use of biomass is contributing to net reductions of atmospheric carbon depends on carbon losses during and following harvest, rates of forest regrowth, and the use of harvested wood and carbon retention in long-lived or short-lived products, as well as the emission reductions achieved through the substitution of emissions-intensive products with wood products (Lemprière et al. 2013 <sup>[[#fn:r1097|1097]]</sup> ; Lundmark et al. 2014 <sup>[[#fn:r1098|1098]]</sup> ; Xu et al. 2018b <sup>[[#fn:r1099|1099]]</sup> ; Olguin et al. 2018 <sup>[[#fn:r1100|1100]]</sup> ; Dugan et al. 2018 <sup>[[#fn:r1101|1101]]</sup> ; Chen et al. 2018b <sup>[[#fn:r1102|1102]]</sup> ; Pingoud et al. 2018 <sup>[[#fn:r1103|1103]]</sup> ; Seidl et al. 2007 <sup>[[#fn:r1104|1104]]</sup> ). Studies that ignore changes in forest carbon stocks (such as some lifecycle analyses that assume no impacts of harvest on forest carbon stocks), ignore changes in wood product pools (Mackey et al. 2013 <sup>[[#fn:r1105|1105]]</sup> ) or assume long-term steady state (Pingoud et al. 2018 <sup>[[#fn:r1106|1106]]</sup> ), or ignore changes in emissions from substitution benefits (Mackey et al. 2013 <sup>[[#fn:r1107|1107]]</sup> ; Lewis et al. 2019 <sup>[[#fn:r1108|1108]]</sup> ) will arrive at diverging conclusions about the benefits of SFM. Moreover, assessments of climate benefits of any mitigation action must also consider the time dynamics of atmospheric impacts, as some actions will have immediate benefits (e.g., avoided deforestation), while others may not achieve net atmospheric benefits for decades or centuries. For example, the climate benefits of woody biomass use for bioenergy depend on several factors, such as the source and alternate fate of the biomass, the energy type it substitutes, and the rates of regrowth of the harvested forest (Laganière et al. 2017 <sup>[[#fn:r1109|1109]]</sup> ; Ter-Mikaelian et al. 2014 <sup>[[#fn:r1110|1110]]</sup> ; Smyth et al. 2017 <sup>[[#fn:r1111|1111]]</sup> ). Conversion of primary forests in regions of very low stand-replacing disturbances to short-rotation plantations where the harvested wood is used for short-lived products with low displacement factors will increase emissions. In general, greater mitigation benefits are achieved if harvested wood products are used for products with long carbon retention time and high displacement factors.

With increasing forest age, carbon sinks in forests will diminish until harvest or natural disturbances, such as wildfire, remove biomass carbon or release it to the atmosphere (Seidl et al. 2017 <sup>[[#fn:r1112|1112]]</sup> ). While individual trees can accumulate carbon for centuries (Köhl et al. 2017 <sup>[[#fn:r1113|1113]]</sup> ), stand-level carbon accumulation rates depend on both tree growth and tree mortality rates (Hember et al. 2016 <sup>[[#fn:r1114|1114]]</sup> ; Lewis et al. 2004 <sup>[[#fn:r1115|1115]]</sup> ). SFM, including harvest and forest regeneration, can help maintain active carbon sinks by maintaining a forest age-class distribution that includes a share of young, actively growing stands (Volkova et al. 2018 <sup>[[#fn:r1116|1116]]</sup> ; Nabuurs et al. 2017 <sup>[[#fn:r1117|1117]]</sup> ). The use of the harvested carbon in either long-lived wood products (e.g., for construction), short-lived wood products (e.g., pulp and paper), or biofuels affects the net carbon balance of the forest sector (Lemprière et al. 2013 <sup>[[#fn:r1118|1118]]</sup> ; Matthews et al. 2018 <sup>[[#fn:r1119|1119]]</sup> ). The use of these wood products can further contribute to GHG emission-reduction goals by avoiding the emissions from the products with higher embodied emissions that have been displaced (Nabuurs et al. 2007 <sup>[[#fn:r1120|1120]]</sup> ; Lemprière et al. 2013 <sup>[[#fn:r1121|1121]]</sup> ). In 2007 the IPCC concluded that ‘[i]n the long term, a sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual sustained yield of timber, fibre or energy from the forest, will generate the largest sustained mitigation benefit’ (Nabuurs et al. 2007 <sup>[[#fn:r1122|1122]]</sup> ). The apparent trade-offs between maximising forest carbon stocks and maximising ecosystem carbon sinks are at the origin of ongoing debates about optimum management strategies to achieve negative emissions (Keith et al. 2014 <sup>[[#fn:r1123|1123]]</sup> ; Kurz et al. 2016 <sup>[[#fn:r1124|1124]]</sup> ; Lundmark et al. 2014 <sup>[[#fn:r1125|1125]]</sup> ). SFM, including the intensification of carbon-focused management strategies, can make long-term contributions towards negative emissions if the sustainability of management is assured through appropriate governance, monitoring and enforcement. As specified in the definition of SFM, other criteria such as biodiversity must also be considered when assessing mitigation outcomes (Lecina-Diaz et al. 2018 <sup>[[#fn:r1126|1126]]</sup> ). Moreover, the impacts of changes in management on albedo and other non-GHG factors also need to be considered (Luyssaert et al. 2018 <sup>[[#fn:r1127|1127]]</sup> ) (Chapter 2). The contribution of SFM for negative emissions is strongly affected by the use of the wood products derived from forest harvest and the time horizon over which the carbon balance is assessed. SFM needs to anticipate the impacts of climate change on future tree growth, mortality and disturbances when designing climate change mitigation and adaptation strategies (Valade et al. 2017 <sup>[[#fn:r1128|1128]]</sup> ; Seidl et al. 2017 <sup>[[#fn:r1129|1129]]</sup> ).

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